(The following NASA Educational Brief, entitled "The Gamma Ray Observatory," EB-102/1-91, is distributed by NASA Headquarters, Washington, D.C.)


Space Astronomy

The science of astronomy deals with the questions that we humans ask about ourselves and our place in the universe. Through astronomy, we seek to learn how the universe began, how it evolved to its present state, and how it will change in the future. In the process, we also learn about our own origins and the origin of our planet. With a better understanding of our place in the universe, we can hope to appreciate the beauty and fragility of our Earth and of our species.

For most of human history we have been limited in our study of the universe by our eyesight and by the Earth's atmosphere. Because the visible light that our eyes can see represents only a tiny portion of the complete electromagnetic spectrum, studying the universe in visible light is a bit like studying human history by concentrating on just a single century. Only by expanding our view into invisible wavelengths of light‹radio, microwaves, infrared, (visible), ultraviolet, X-rays, and gamma rays‹can we hope to get a complete picture of our universe.

Only light from the visible and radio portions of the electromagnetic spectrum, plus a few selected bands of infrared light, can penetrate Earth's atmosphere to reach the ground. To study the universe in other wavelengths, and to overcome atmospheric distortion of visible light, the field of space astronomy was born, in which telescopes and instruments are carried beyond the limitations imposed by our atmosphere. Since its beginnings more than 30 years ago, space astronomy has revolutionized our understanding of the universe, enabling us to discover dozens of new types of objects that go unseen in studies restricted to visible light.

The Observatory

The Gamma Ray Observatory (GRO) is a space-based instrument designed to study the universe in an invisible, high-energy form of light known as gamma rays. Although a variety of smaller satellites and high-altitude balloons have carried instruments to study the universe in gamma-ray light during the past 30 years, GRO represents a dramatic improvement in sensitivity (the ability to detect faint sources), spectral range (the ability to detect gamma rays of many different energies), and resolution (the ability to locate the direction from which detected gamma rays originate).

The GRO is the second of four Great Observatories being built by NASA to study the universe across the electromagnetic spectrum. The first, the Hubble Space Telescope, was launched in 1990; it primarily studies visible and ultraviolet light. The third and fourth Great Observatories, which will detect X-rays and infrared, respectively, are planned for later in this decade.


Weighing over 15 metric tons (35,000 pounds), GRO, which carries four distinct instruments for detecting gamma-rays, will be the heaviest scientific satellite launched by the Space Shuttle into low Earth orbit. The large masses of its instruments are necessary to ensure that we detect a significant number of photons, in a reasonable amount of time, because the numbers of gamma rays coming from most celestial sources are small. GRO will orbit Earth every 93 minutes, at an altitude of 450 kilometers, and is designed to operate for a minimum of 2 years.

What are Gamma Rays?

Gamma rays are a form of light that cannot be seen by the human eye. Gamma rays have the highest energies (and, therefore, the highest frequencies and shortest wavelengths) of any type of light radiation. Since high-energy processes tend to produce high-energy radiation, gamma rays are emitted by some of the most exotic structures in our universe‹exploding stars (supernovae), neutron stars, black holes, and quasars. The study of gamma rays offers us a window into the inner workings of these and other fascinating objects, providing insights unattainable from the study of any other form of radiation.

Just as we know that there are sounds that we cannot hear (e.g., we cannot hear the sound of a dog whistle), there are many forms of light that we cannot see. In fact, the visible light that our eyes can see is only a very tiny part of the complete electromagnetic spectrum. Although all forms of electromagnetic radiation* travel through space at the same speed, individual ~pieces" of light (called photons) differ from one another in wavelength, frequency, and energy. When studying gamma rays, astronomers, prefer to think in terms of the energy of the radiation, which they measure in units called electron-volts, abbreviated eV. (The eV is a standard unit of energy in science.)

*The term "radiation" simply refers to any energy form or particles traveling through space. Thus, not all radiation is dangerous, as even the visible light that we see is one form of radiation!

The Gamma-Ray Universe

What do we hope to learn by studying the universe in gamma-ray light?

Each portion of the electromagnetic spectrum reveals a unique view of the universe, providing information unavailable from other parts of the spectrum. Gamma rays are particularly important for probing rare, dynamic objects generating intense fields of energy.

In particular, GRO will enable to us to study gamma rays produced by (l) solar flares, powerful eruptions on the surface of the Sun that can affect us directly on Earth; (2) nuclear reactions in supernovae (explosions of stars); (3) the decay of radioactive nuclei in interstellar space; (4) the interaction of high-energy particles, called cosmic rays, with interstellar material; (5) energetic reactions, some involving the production and subsequent annihilation of matter and antimatter, near neutron stars (including the rapidly rotating pulsars) and black holes; (6) energetic reactions in the cores of galaxies, including those in the incredibly powerful, distant quasars; and (7) the mysterious gamma-ray bursters, objects of which we know little beyond that they can produce, for very short periods of time, as much gamma-ray energy as trillions of solar flares.


E = mc2 (E = energy; m = mass; c = speed of light)

Einstein's famous formula, enunciated in his theory of relativity, tells us that mass is simply one form of energy. Today we know that, under certain circumstances, mass can be converted into other forms of energy, and vice versa. Nuclear fission (the energy source for nuclear power plants) and nuclear fusion (the energy source for the Sun) are common examples of processes that convert small amounts of mass into energy. Under conditions of extreme energy, tiny amounts of mass can simply "pop" into existence. Such is the case in the giant particle accelerators built by physicists, and around many exotic cosmic structures. Whenever mass is produced, it always comes in pairs of particles: one particle of matter, and one of antimatter. Each particle and its antiparticle are identical except that certain physical properties, like electric charge, are exactly opposite. Antimatter usually doesn't last long; as soon as an antiparticle meets up with a corresponding matter particle, the pair mutually annihilate, turning their mass into gamma rays detectable by GRO.

Despite the rarity of gamma rays, their exotic producers are intimately tied to our own existence. For example, gamma-ray observations help us to confirm the belief that all of the elements from which we, and our planet, are made (except for hydrogen and helium) were produced by stars that died in supernovas. As another example, by studying the highest energy processes occurring in our universe, we may learn much more not only about the structures that emit these gamma rays, but also about the processes that may have taken place shortly after the universe began in the energetic Big Bang, some 10 to 20 billion years ago. Thus, our study of the gamma-ray universe promises to teach us much about our own origins.

How Do We Detect Gamma Rays?

Astronomers seeking to study the gamma-ray universe are faced with two major difficulties. First, gamma rays do not penetrate Earth's atmosphere, and therefore cannot easily be studied from the ground. GRO will overcome this difficulty by its location above our atmosphere, in Earth orbit. Second, because of their high energies, gamma rays cannot be reflected or focused like other forms of light. Thus, although the instruments of GRO are essentially telescopes for seeing gamma-ray light, they do not look at all like ordinary telescopes. Instead, the GRO instruments observe gamma rays indirectly, by monitoring flashes of visible light, called scintillations, that occur when gamma rays strike the detectors (made of liquid or crystal materials) built into the instruments.

The Gamma Ray Observatory Spacecraft

The gamma rays emitted from celestial objects span a wide range of energies; the most energetic gamma rays to be studied by GRO have energies some 1 million times greater than the weakest. This is a far greater range in energy than that spanned by visible light, and no single instrument yet devised can detect gamma rays throughout this range. GRO carries four distinct instruments that together span the gamma-ray range from about 20,000 to 30 billion eV. (An electron volt is the energy gained by one electron when accelerated through an electric potential of one volt.) Each of the four instruments has a unique design, and is specialized for particular types of observations:

The Burst and Transient Source Experiment (BATSE) is designed to search the sky for "bursts" of gamma rays that may occur unexpectedly from any direction. BATSE consists of eight identical detectors, located at each vertex of the spacecraft, to give it a very wide field of view (i.e., it can study a large part of the sky simultaneously), and it can measure rapid changes in the intensity of gamma-ray light. BATSE works in the low-energy part of the gamma ray range (20,000 to 2 million eV) in which bursts are expected. Once BATSE discovers a burst of gamma rays, it can signal the other three instruments to study the source in more detail.

The Oriented Scintillation Spectrometer Experiment (OSSE) also is optimized to low-energy gamma rays (100,000 to 10 million eV), but can obtain higher quality spectra than BATSE. One function of OSSE will be to respond to signals discovered by BATSE, allowing their sources to be studied in further detail. Of particular importance, OSSE is sensitive to the spectral line produced (at 511,000 eV) when an electron and its antimatter counterpart, the positron, collide and mutually annihilate each other. Because antimatter production and annihilation is an important process near neutron stars (and pulsars) and black holes, study of this line can provide valuable insights into these bizarre structures.

The Imaging Compton Telescope (COMPTEL) is designed for observations at moderate gamma-ray energies (l to 30 million eV). Because COMPTEL has a wide field of view (though not as wide as BATSE) and can accurately locate gamma ray sources, one of its primary functions will be to produce a detailed map of the sky as seen in moderate gamma rays. It is expected that this survey will reveal many new sources of gamma ray emissions.

The Energetic Gamma Ray Experiment Telescope (EGRET) detects high-energy gamma rays (20 million to 30 billion eV). One of its primary missions will be to generate a map of the sky as seen in high-energy gamma rays, complementing the map produced by COMPTEL. Another will be to discover and monitor gamma-ray emissions from pulsars.


Vocabulary Terms

Ask your students to look up any or all of the following terms. Some of the terms, in themselves, would make suitable topics for research papers.

antimatter, astronomy, black hole, cosmic rays, detectors, electromagnetic spectrum, electron-volt, energy, frequency, gamma-ray bursters, gamma rays, Great Observatories, neutron star, nuclear fission, nuclear fusion, particle accelerators, positron, pulsar, quasars, radiation, relativity (theory), resolution, scintillation, sensitivity, space astronomy, spectral range, supernova, wavelength

Discussion Topics

1. Discuss the advantages of space astronomy and the difficulties of working with orbiting observatories. In considering the advantages, remember that many students mistakenly believe that observatories in Earth orbit are "closer to the stars." In fact, the altitude of orbiting spacecraft is negligible even in comparison with distances in our own solar system. The orbital altitude of GRO, 450 km, is only about the distance between Washington, D.C. and New York City. (Orbiting observatories are advantageous simply because they are not affected by our atmosphere.) Difficulties to consider include: cost; impacts by meteors or orbiting space debris; maintenance or repair of facilities; and pointing telescopes accurately without a foundation on solid ground.

2. Discuss how the need for studying light from across the electromagnetic spectrum is similar to our need to use all of our senses to understand our surroundings. You might ask your students to try observing the happenings around them using only their sense of smell, hearing, sight, etc. What difficulties would be presented? How does this relate to the problems facing astronomers?

3. High energy radiation, X-rays and gamma rays, are used today in medical research as well as in other technical fields. Ask some of your students to do a research paper about the uses of this radiation in a field other than astronomy.

4. To develop our understanding of cosmology (the origin and evolution of the universe), high-energy astronomy is used hand-in-hand with subatomic physics. Physicists use giant particle accelerators to probe the universe at minute scales, producing new particles, including antimatter particles, in the process. Ask some of your students to write research papers about particle accelerators, or about how the study of subatomic physics is linked with high-energy astronomy and cosmology.

Bibliography 1. "The Great Annihilator," G. Taubes, DISCOVER, June 1990, pp. 68-72.

2. "Gamma-Ray Bursts," SCIENTIFIC AMERICA, May 1989, p. 26.

3. "Gamma-Ray Observatory to Study Celestial Forces that Shaped the Universe," B.A. Smith, AVATION WEEK AND SPACE TECHNOLOGY, May 5, 1990, pp. 70-73.

4. "Seeking the Origins of Cosmic Rays," D.H. Smith, SKY AND TELESCOPE, May 1990, pp. 479-484.

5. NATIONAL GEOGRAPHIC PICTURE ATLAS OF OUR UNIVERSE, R.A. Galant, National Geographic Society, Washington, D.C., 1986, pp. 284.

6. "The Great Supernova of 1987," S. Woosley and T. Weaver, SCIENTIFIC AMERICAN, August 1989, pp. 32-40.

7. "Supernovae," H.A. Bethe, PHYSICS TODAY, September 1990, pp. 24-27.

8. "What are Gamma-Ray Bursters?," K Hurley, SKY AND TELESCOPE, August 1990, pp. 143-147.

U.S. Governrnent Printing Office: 1991‹521-561